In recent years, it has been proposed to use a solar energy as an alternative energy that substitutes for the thermal or the hydraulic power. Therefore, expectation for a solar cell constructed by a photoelectric conversion element for converting the solar energy into an electric energy becomes very great.
Under the circumstances, there have been proposed various types of solar cells or photoelectric conversion elements, such as silicon-based, compound-based, and organic-based solar cells or photoelectric conversion elements.
Further, among the solar cells of these types, the silicon-based solar cells use, as their material, silicon which is an abundant resource on earth. Therefore, it is considered that the problem of resource exhaustion or the like does not arise as compared with the compound-based and the organic-based solar cells as the other types.
Among the silicon-based solar cells, in a case of an amorphous type silicon solar cell, a film thickness of an amorphous silicon (a-Si) film can be reduced to 1/100 or less as compared with monocrystalline type and polycrystalline type silicon solar cell. Therefore, the amorphous type is suitable for actually manufacturing high-power and large-area solar cell at a low cost.
However, the amorphous type silicon solar cell has an energy conversion efficiency of about 6% which is significantly low as compared with the monocrystalline type and the polycrystalline type silicon solar cells having an energy conversion efficiency of about 20%. In addition, it is pointed out that the energy conversion efficiency of the amorphous type silicon solar cell decreases with an increase of its area.
The present inventors previously proposed, in Patent Document 1, an amorphous type silicon solar cell or photoelectric conversion element having an energy conversion efficiency exceeding 6%. The proposed amorphous type silicon solar cell or photoelectric conversion element includes a first electrode layer formed of a transparent electrode, a second electrode layer, and one or a plurality of power generation laminates provided between the first and the second electrode layers. The power generation laminate has a so-called nip structure comprising an n-type amorphous semiconductor layer (in particular, n-type amorphous silicon layer) formed in contact with the first electrode layer, a p-type amorphous semiconductor layer (in particular, p-type amorphous silicon layer) formed in contact with the second electrode layer, and an i-type semiconductor layer (i-type silicon layer) provided between the n-type amorphous semiconductor layer and the p-type semiconductor layer.
In order to increase the conversion efficiency, it is also proposed to use a light-emitting laminate of a nip structure formed of microcrystalline silicon (μC-Si) which consumes a relatively small amount of silicon (Patent Document 2).
Further, the amorphous type solar cell or photoelectric conversion element described in Patent Document 1 adopts, as the first electrode layer brought into contact with the n-type amorphous silicon layer as the n-type amorphous semiconductor layer, a transparent electrode using n+-type ZnO with a low energy barrier.